The present invention relates to a catalyst-adsorbent for purification of exhaust gases and a method for purification of exhaust gases using the catalyst-adsorbent. More particularly, the present invention relates to a catalyst-adsorbent for purification of exhaust gases, capable of effectively purifying harmful substances present in exhaust gases, particularly hydrocarbons generated in a large amount during the cold start of an internal combustion engine, as well as to a method for purification of exhaust gases using the catalyst-adsorbent.
Various catalysts have heretofore been proposed for purification of harmful components [e.g. hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx)] present in exhaust gases emitted from automobiles, and the like. For example, three-way catalysts containing Pd showing an excellent low-temperature light-off performance, in a relatively high concentration as the only noble metal component showing a catalytic activity, are proposed in SAE Paper Nos. 941058 and 930386.
Also, zeolite-containing catalysts are disclosed in Japanese Patent Application Kokai (Laid-Open) Nos. 293384/1993 and 174937/1990, and Japanese Patent Application No. 305429/1990.
In order for a catalyst to exhibit its catalytic activity, the catalyst must be heated to a given temperature or higher. Therefore, during the cold start of an automobile engine when the catalyst provided in the exhaust gas system is not heated sufficiently, the harmful components of the exhaust gas are discharged into the air without being purified. While regulations on the harmful components present in exhaust gases, particularly HC are becoming more strict, HC is generated in a large amount during the cold start. Hence, it is an important technical task to control the discharge of HC into air during the cold start.
In this connection, attention has recently been paid to a technique of utilizing the zeolite adsorptivity for HC and allowing a zeolite-containing adsorbent to adsorb the HC generated during cold start of an engine, from the start of the engine to a time when the catalyst active component is heated sufficiently.
When there are used, of the above-mentioned techniques, the catalysts proposed by SAE Paper Nos. 941058 and 930386, containing no zeolite as an adsorbent although containing Pd of excellent-light off performance in a relatively high concentration, the HC, which is generated in a large amount during cold start of the engine, is discharged into the air without being purified, up to the time when the catalyst is activated. Further, since the catalysts contain no adsorbent, no optimization is employed to effectively purify a high concentration of the HC generated when the HC adsorbed by an adsorbent is desorbed from the adsorbent.
In the catalyst disclosed in Japanese Patent Application No. 305429/1990, zeolite is used merely as a substrate for loading a noble metal and is not optimized as an adsorbent; therefore, the catalyst has an insufficient adsorptivity. Moreover, the catalyst contains Pd in a low concentration, is insufficient in light-off performance and purification ability, and is unable to effectively purify a high concentration of the HC desorbed from the adsorbent as the engine warms up.
In the catalysts disclosed in Japanese Patent Application Kokai (Laid-Open) No. 293384/1993, no optimization (in presence or absence of Pd, Pd concentration, etc.) is made to effectively purify a high concentration of the HC-desorbed from zeolite; therefore, the catalyst is insufficient in light-off performance and purification ability.
In the catalyst system disclosed in Japanese Patent Application Kokai (Laid-Open) No. 174937/1990, zeolite is used as a substitute for Rh to reduce the amount of Rh (which is an expensive noble metal very small in ore reserve) and no adsorbent optimization is made; therefore, the catalyst has an insufficient adsorptivity. Moreover, the catalyst contains a low concentration of Pd, is insufficient in light-off performance and purification ability, and is unable to effectivley purify a high concentration of the HC desorbed from the adsorbent as the engine warms up.
The present invention has been made in view of the above-mentioned problems of the prior art and is intended to provide (1) a catalyst-adsorbent wherein zeolite can effectively act as an adsorbent for HC and the concentrations of catalyst active components, etc. are optimized so that a high concentration of the HC desorbed from the adsorbent with the engine warms up can be effectively purified, and (2) a method for effective purification of exhaust gases using the catalyst-adsorbent (1).
According to the present invention there is provided a catalyst-adsorbent for purification of exhaust gases, comprising a monolithic carrier and a catalyst-adsorbent layer formed thereon, the catalyst-adsorbent layer comprising a catalyst for reduction of the carbon monoxide, hydrocarbons and nitrogen oxides emitted from internal combustion engines and an adsorbent for reduction of the hydrocarbons emitted during cold start of said engines, the catalyst being composed mainly of catalyst particles each comprising a heat-resistant inorganic oxide and at least one noble metal selected from Pt, Pd and Rh, loaded thereon, the catalyst containing at least catalyst particles each comprising a heat-resistant inorganic oxide and 2-30% by weight, based on said oxide, of Pd loaded thereon, the adsorbent comprising adsorbent particles composed mainly of zeolite.
According to the present invention, there is also provided a method for purification of exhaust gases, which comprises providing a catalyst-adsorbent in an exhaust gas system of an internal combustion engine and conducting exhaust gas purification while introducing secondary air into the exhaust gas system at a site upstream of the catalyst-adsorbent for a certain length of time during cold start of the engine, in which the catalyst-adsorbent comprises a monolithic carrier and a catalyst-adsorbent layer formed thereon, the catalyst-adsorbent layer comprising a catalyst for reduction of the carbon monoxide, hydrocarbons and nitrogen oxides emitted from internal combustion engines and an adsorbent for reduction of the hydrocarbons emitted during cold start of said engines, the catalyst being composed mainly of catalyst particles each comprising a heat-resistant inorganic oxide and at least one noble metal selected from Pt, Pd and Rh, loaded thereon, the catalyst containing at least catalyst particles each comprising a heat-resistant inorganic oxide and 2-30% by weight, based on said oxide, of Pd loaded thereon, the adsorbent comprising adsorbent particles composed mainly of zeolite.
According to the present invention there is further provided a method for purification of exhaust gases, which comprises providing a catalyst-adsorbent in an exhaust gas system of an internal combustion engine and conducting exhaust gas purification while regulating amounts of combustion air and fuel for a certain length of time during cold start of the engine to shift the composition of the exhaust gas to a lean side, in which method the catalyst-adsorbent comprises a monolithic carrier and a catalyst-adsorbent layer formed thereon, the catalyst-adsorbent layer comprising a catalyst for reduction of the carbon monoxide, hydrocarbons and nitrogen oxides emitted from internal combustion engines and an adsorbent for reduction of the hydrocarbons emitted during cold start of said engines, the catalyst being composed mainly of catalyst particles each comprising a heat-resistant inorganic oxide and at least one noble metal selected from Pt, Pd and Rh, loaded thereon, the catalyst containing at least catalyst particles each comprising a heat-resistant inorganic oxide and 2-30% by weight, based on said oxide, of Pd loaded thereon, the adsorbent comprising adsorbent particles composed mainly of zeolite.